As specified in Third Generation Partnership Project Technical Specification (3GPP TS) 36.331, Radio Resource Control, v. 12.1.0, some of the available subframes within a finite number of consecutive radio frames (or system frames) are allocated to Multimedia Broadcast Multicast Service (MBMS), when the MBMS service is enabled. This MBMS subframe allocation can be repeated periodically. The periodicity can be set to one, two, four, eight, sixteen or thirty-two radio frames. The standard further specifies ways to extend the periodicity to 64, 128 and 256 radio frames. These defined procedures give flexibility to the operator to adjust the bandwidth allocation for MBMS services. Normally MBMS subframe allocation is semi-statically configured. The MBMS subframe pattern is indicated in a System Information Block 2 (SIB2) broadcast message as part of the Multicast Broadcast Single-Frequency Network (MBSFN) SubframeConfiguration Information Element (IE). MBMS service can be configured on any subframes except on subframes 0, 4, 5 and 9 for Frequency Division Duplex (FDD) or subframes 0, 1, 2, 5 and 6 for Time Division Duplex (TDD) within a radio frame. The percentage of the available bandwidth (or radio resources) that can be allocated to MBMS service is limited to 60. These evolved MBMS (eMBMS) subframes (or MBSFN subframes) are only available for eMBMS transmissions once these radio resources are allocated for broadcast/multi-cast services, i.e., they typically can't be used for unicast traffic like for example Voice over Long Term Evolution (VoLTE) or File Transfer Protocol (FTP) for non Transmission Mode 9 (TM9) & TM10 UEs. Even for TM9 and TM10 UEs, once radio resources are dimensioned properly, there will not be any un-used resources within the eMBMS subframes available for unicast DL transmission. UEs, which are not interested in the multicast service, listen to the SIB2 messages but may and skip the MBMS subframes by just reading the Physical Downlink Control Channel (PDCCH).
Discontinuous Reception—DRX
User equipment manufacturers and system service providers seek arrangements to maximize the battery life of user equipment. One method for doing so is through the use of discontinuous reception (DRX). As is shown in FIG. 1, discontinuous reception (DRX) reduces battery consumption in the user equipment (UE) by limiting the time when receptions need to be monitored by the UE. The UE can only be scheduled in the Downlink (DL) when the UE monitors the PDCCH. This implies that the UE can only be scheduled during periods of time when the UE is awake (also known as the “active time”, “wake time” or “on duration”). In LTE networks, UEs normally use a timer referred to as the “onDuration” timer to determine when it needs to be awake. Of note, in some scenarios, UEs may additionally also use other timers to track the wake time, e.g., the DRX inactivity timer, DRX retransmission timer and/or Medium Access Control (MAC) contention resolution timer. It is advantageous to spread the wake time of the UEs in order to spread the load due to scheduling in time. This means that the Evolved Node B (eNB) base station attempts to configure the DRX for UEs so that the UEs do not have simultaneous wake times.
Voice Over IP for Long Term Evolution (VoLTE):
In LTE, all packets are delivered using the Internet Protocol (IP). This also applies to traditional circuit switched services which make use of scheduling using IP. This is called Voice over IP (VoIP). In a typical VoIP arrangement, a voice encoder on the transmitter side encodes the speech into packets with atypical speech duration of 20 ms. Voice over LTE (VoLTE) enables LTE networks to provide voice services. The mouth-to-ear delay introduced by the transport scheduling and transmission of the VoLTE packets is one of the main factors determining the experienced speech quality. This necessitates a relatively tight delay budget for VoLTE to ensure good speech quality. Up to the limit of the delay budget, the speech quality is usually acceptable. This means that it is generally sufficient to schedule a VoLTE service once every 40 ms and bundle two packets. Such a scheduling method allows for a good balance between efficient resource usage and sufficient speech quality.
VoLTE & DRX
To conserve the battery power of the UE, VoLTE users may operate with DRX enabled. Typically, the DRX period is set to 20 ms without packet bundling or 40 ms with packet bundling with the DRX ON time (also referred to as active time or wake time) set to more than one subframe. Further, when the DRX is enabled, the connected UEs monitor PDCCH while it is awake, i.e., while the onDuration timer, DRX inactivity timer, DRX retransmission timer and/or MAC contention resolution timer is running.
VoLTE, DRX & eMBMS
When eMBMS co-exists with VoLTE, the available subframes for DL transmission of VoIP packets are reduced by eMBMS. Further, when DRX is enabled, the connected UEs do not monitor PDCCH continuously but only during the wake time.
Since both DRX and eMBMS reduce the available subframes where the UE can be scheduled in DL, a combination of these features can reduce the possible subframes available for downlink transmission to one or even zero subframes. The eNB then needs to wait for another opportunity to schedule the UE, i.e., in some instances, the scheduling of DL packets to the UE may be delayed. As noted above, for VoLTE, packet delay is an important factor determining the perceived quality. The aforementioned problem can reduce the VoLTE performance even when only a small number of VoLTE UEs are present in the cell, i.e. it is a problem that not only occurs at high load but also during small network loads. Although the voice quality can be partially recovered if DRX is disabled for the VoLTE Radio Access Bearers (RABs), the improvement would be obtained at the expense of battery drain at the VoLTE UEs, which is undesirable.